Aberrometers measure the refraction of a subject's eye using infrared light.
Optical coherence tomography (OCT) is employed to measure the distances between different surfaces within an eye. Various types of OCT are known, including Fourier domain optical coherence tomography (FD-OCT), which in turn includes spectral domain optical coherence tomography (SD-OCT) and swept-source OCT (SS-OCT).
In SS-OCT, a swept (scanning) laser may be employed to produce a laser signal with a linear frequency ramp or “chirp.” The swept laser signal is applied to a reference arm or reference path of an OCT interferometer, and also through a sample arm or sample path of the OCT interferometer to an object (e.g., an eye) which is to be measured. Reflections of the laser signal returned from the different surfaces of structures within the object (e.g., eye) in the sample path can be combined with the laser signal output by the reference path to produce corresponding OCT peaks, corresponding to the depths of the different reflection and scattering surfaces within the object being measured, in an OCT signal output by a detector.
The accuracy of the measurements depend on a number of factors, including the resolutions or “widths” of the peaks in the OCT signal produced by the different reflection and scattering surfaces within the object being measured. Meanwhile, the widths of the peaks in the OCT signal are affected by the total amount of group delay dispersion that is present due to the combination of the group velocity dispersion characteristics of the optical fibers and the optical system that comprise the OCT interferometer. In particular, any difference in group delay dispersion between the sample path and the reference path of the OCT interferometer can lead to significant broadening of the OCT peaks on the OCT output signal, which can affect the accuracy of the measurements.
It would therefore be desirable to reduce or minimize any difference in group delay dispersion differences between the sample arm and the reference arm of an OCT interferometer.